Localized Bathless Metal-Composite Plating via Electrostamping

Summary

Presented here is a protocol of bathless electroplating, where a stagnant metal salt paste containing composite particles are reduced to form metal composites at high loading. This method addresses the challenges faced by other common forms of electroplating (jet, brush, bath) of embedding composites particles into the metal matrix.

Abstract

Composite plating with particles embedded into the metal matrix can enhance the properties of the metal coating to make it more or less conductive, hard, durable, lubricated or fluorescent. However, it can be more challenging than metal plating, because the composite particles are either 1) not charged so they do not have a strong electrostatic attraction to the cathode, 2) are hygroscopic and are blocked by a hydration shell, or 3) too large to remain stagnate at the cathode while stirring. Here, we describe the details of a bathless plating method that involves anode and cathode nickel plates sandwiching an aqueous concentrated electrolyte paste containing large hygroscopic phosphorescent particles and a hydrophilic membrane. After applying a potential, the nickel metal is deposited around the stagnant phosphor particles, trapping them in the film. The composite coatings are characterized by optical microscopy for film roughness, thickness and composite surface loading. In addition, fluorescence spectroscopy can be used to quantify the illumination brightness of these films to assess the effects of various current densities, coating duration and phosphor loading.

Introduction

Traditional electroplating is widely used to deposit thin films of a variety of metals, alloys, and metal-composites onto conductive surfaces to functionalize them for the intended application^{1,2,3,4,5,6,7,8,9,10,11,12}. This method adds a metal finish to parts used in the manufacturing of aerospace, automotive, military, medical, and electronic equipment. ­­­­The object to be plated, the cathode, is submerged in an aqueous bath containing metal salt precursors, which are reduced to metal at the surface of the object by the application of a chemical or electrical potential. Non-charged composite particles can be incorporated into the metal film by adding these to the bath during coating to enhance the film properties for increased hardness in the case of metal oxides and carbides, smoothness with polymers or lubrication with liquid oils^12,13. However, because these particles lack an inherent attraction to the cathode, the ratio of composite that is incorporated into the metal remains low for bath plating^13,14,15. This is especially problematic for large particles that do not adsorb to the cathode long enough to be embedded by the growing metal film. Additionally, hygroscopic particles solvate in aqueous solutions and their hydration shell acts as a physical barrier impeding contact with the cathode¹⁶.

Some promising methods have been shown to mitigate this effect by using dry non-polar solvents to remove the hydration barrier completely¹⁷, or by decorating the composite particles with charged surfactant molecules¹⁶ that disrupt the hydration shell to allow contact between the particle and the cathode. However, because these methods involve organic materials, carbon contamination is possible in the film and breakdown of these organic materials could occur at the electrodes. For example, the organic solvents used (DMSO₂ and acetamide) are heated to 130 °C in an inert atmosphere for air-free coating; however, we found them to be unstable during coating in air. Due to resistive heating at the electrodes, redox reactions with organic materials may result in impurities or sites for heterogeneous nucleation and growth of metal nanoparticles¹⁸. As a result, there is a need for an organic-free aqueous electroplating method that addresses the long-standing challenge of particle-cathode adsorption. So far, metal-composite bath coating has been shown to embed particles up to a few micrometers in diameter¹⁹ and as high as 15 % loading^16,17.

In response to this, we describe an inorganic bathless electrostamping method that forces composite particles to become embedded into the film at high surface coverages despite their large size and hygroscopic nature²⁰. By removing the bath, the process does not involve containers of hazardous coating liquids and the object to be plated does not need to be submerged. Therefore, large, cumbersome or otherwise corrosion- or water-sensitive objects, can be plated or “stamped” in select areas with the composite material. In addition, the removal of excess water requires less clean-up of liquid hazardous waste.

Here, we demonstrate this method to produce bright fluorescent metal films by co-depositing non-toxic and air-stable europium and dysprosium doped, strontium aluminate (87 ± 30 µm) with nickel at high loadings (up to 80%). This comes in contrast to previous examples that were plated in a bath and therefore were limited to small (nanometers to a few micrometers) phosphors¹². In addition, previously reported electrodeposited films fluoresce only under short-wave UV-light, with the exception of a recent report that grew 1 – 5 µm luminescent strontium aluminate crystals in an alumina film with plasma electrolyte oxidation²¹. Fluorescent metal films could have far-reaching applications in many industries involving dim-light environments including road sign illumination²¹, aircraft maintenance equipment location and identification²⁰, automobile and toy decorations, invisible messages, product authentication²², safety lighting, mechanochromic stress identification¹⁰ and tribological wear visual inspection^12,16. Despite these potential uses for glowing metal surfaces, this method could also be expanded to include additional large and/or hygroscopic composite particles to produce a new variety of metal-composite functional coatings that were previously not possible via electroplating.

Protocol

1. Preparing coating salts

CAUTION: Nickel salts and boric acid are toxic and should be handled with proper personal protective equipment including nitrile gloves, goggles and a lab coat. Strong acids and bases should be handled in the fume hood, and all waste chemicals should be disposed of as hazardous waste.

1. Using a balance, weigh out the following powders in these ratios: 10.000 g of NiSO₄·6H₂O, 2.120 g of NiCl₂·6H₂O, 1.600 g of H₃BO₃ and combine in a vial together. See Table 1 for concentrations.
2. Weigh out 1.800 g of SrAl₂O₄:Eu²⁺, Dy³⁺ phosphor or alternative phosphors including europium doped yttrium oxide, europium doped barium magnesium aluminate, or replace with alternative oxide, metal, or organic composite material depending on the desired effect.
   NOTE: The amount added here may vary based on the properties of the composite material and the desired qualities of the metal-composite film.
3. Using a porcelain mortar and pestle, grind the composite powder for approximately 10 minutes until it becomes a fine powder.
   NOTE: This does not change the particle size, but does separate aggregated particles.
4. Likewise, grind the salt mixture from step 1.1 in batches until it becomes a fine powder.
5. Combine the ground phosphor with the ground salt mixture in a container for storage.
6. Weigh out 0.188 grams of the mixture per cm² of coating area, as prepared in step 1.5 and add to a container with an open top that is easy to access.
7. To this, add 40 µL of water per cm² of coating area, and stir to partially dissolve the salts forming a thick paste. Set this aside.
   NOTE: The protocol can be paused here.

2. Preparing the electrodes

1. Using scissors, cut the anode to the size and shape that matches the object to be plated. In this example, we prepare a 4 cm² nickel foil to be coated, and a 4 cm² nickel anode is cut to match this.
   NOTE: Other objects can be coated including large objects. In this case, select the area on the object to be coated, and cut out the anode to match the coating area.
2. Using a cotton swab or a cloth, clean the surface of the anode foil and the cathode (coating object surface) with concentrated (10 M) potassium hydroxide or sodium hydroxide base to remove organic material. Next, rinse the surfaces with water to remove excess base.
3. Using a cotton swab or a cloth, activate the object surface with concentrated acid. In the case of nickel, 37% vol/vol HCl is used, although for steel, 10% by volume aqueous HCl may be more appropriate. Please refer to the recommendations for activating metal surfaces provided elsewhere to determine the appropriate method for activating specific metals or alloys^23,24.
   NOTE: After this step, the metal surface is reactive and the surface will start to react with oxygen in the air to form an oxide layer. This will cause the surface to be inactive, so the following steps (2.4 – 3.5) should be performed in the next 5 minutes; otherwise, step 2.3 should be repeated before continuing.
   CAUTION: This step should be performed in a fume hood to avoid exposure to HCl vapors.
4. Quickly, deposit the coating paste onto the cathode object. In this case, the cathode is a 4 cm² nickel foil on the benchtop. Cover the object area to be plated evenly and try to avoid gaps in the paste.
   NOTE: In this example, we are painting on this paste with two scoopulas, however, other options may include spraying, dipping or doctor blading to increase the speed and efficiency of this step.
5. Using a cotton swab or a cloth, activate the anode with concentrated acid by dipping the swab in the acid and gently rubbing the cathode surface. In the case of nickel, 70% vol/vol HNO₃ can be used.
   NOTE: However, other acids may be more appropriate for specific metals and alloys. Please refer to the recommendations provided elsewhere for the appropriate reagent to activate specific anode surfaces^23,24.
   CAUTION: This step should be performed in a fume hood to avoid exposure to NO₂, a toxic brown gas that is formed during the reaction. Continue treating the surface until the surface becomes grey and textured. After this step, the metal surface is reactive and the surface will start to react with oxygen in the air to form an oxide layer, so the following steps should be performed quickly to avoid inactivation of the anode.
6. If calculating the current efficiency is desired, use an analytical balance to record the mass of the anode and the cathode.

3. Assembly and coating

1. Pre-set a power supply to the desired current in constant current mode or voltage, if constant voltage mode is desired. In this example, constant current mode is used with a current of 0.1 Amperes (0.1 A per 4 cm² = 0.025 A/cm²).
   NOTE: For larger or irregularly shaped objects, the coating area can be predetermined with a grid or using a photo with scale bar and an imaging software like ImageJ. The applied current can be scaled to deliver the same current density required for the coating area.
2. Cut a piece of nylon sheet (or alternative hydrophilic membrane) to a size larger than the anode so that the anode does not make direct contact with the cathode object.
   1. Place the nylon sheet on top of the coating paste, and then add a small amount of paste to this.
3. Next, add 1-2 drops of water from a pipette to allow the salt to partially dissolve. Steps 3.2.1 – 3.3 make the nylon sheet conductive and allow for the mass transport of ions through the electrolyte, which is necessary to balance charge in the coating reaction.
4. Finally, add the activated anode on top and attach the negative lead to the cathode object and the positive lead to the anode.
   NOTE: It might be helpful to tape down these leads so that the setup remains stationary, especially if the experiment involves small pieces of metal foil. This is less important for large objects.
5. Cover the system with plastic or seal to help retain water, and apply moderate pressure (~100 g per cm² area), turn on the power supply and continue coating for the desired duration.
6. Turn off the power supply and expose the system.
7. Disconnect the leads, separate the electrodes and rinse the cathode object with water into a waste container.
   1. Soak the other items in water to remove salts and dispose of this aqueous solution in the properly labeled hazardous waste container
   2. Wearing gloves, gently rub the cathode object by hand to remove any uncoated composite particles. The coating is complete and ready for characterization.
   3. Using an analytical balance, record the mass of the anode and the cathode and find the difference between these values and their original mass.
   4. Use Faraday’s Laws of electrolysis to calculate the current efficiency. The theoretical moles of metal coating can be determined using Equation 1.
          Equation 1
      where n is the amount of metal deposited (units: mol), I is the applied current, t is the coating time, F is Faraday’s constant (96485 coulombs per mole) and z is the charge of the metal ion. Calculate this value based on the experimental parameters.
   5. Divide the experimentally determined deposit mass obtained from the masses of the cathode or anode (Steps 2.6 and 3.7.3) by the theoretical mass lost (anode) or gained (cathode) to calculate the current efficiency using Equation 2.
          Equation 2
      [NOTE: At a current efficiency of 100%, under constant voltage, a theoretical deposit mass is expected of approximately 1.095 g of nickel or 12.3 µm of nickel per hour, given 0.04 A and 4 cm² area. Likewise, under constant current, approximately 614.6 µm nickel would theoretically deposit per unit of 1 A∙cm^-2 after 30 min.]

4. Characterization with electrochemistry

1. Use chronopotentiometry to monitor changes in voltage under constant current, and chronoamperometry to monitor changes in current under constant voltage.
   1. Turn on the potentiostat and designate the duration and the applied current or voltage.
   2. Repeat steps 3.2 – 3.5 to prepare the coating.
   3. Use a calibrated 3-electrode system to normalize the voltage to a reference standard.
      1. Place a platinum wire pseudo reference electrode between on top of the nylon sheet and below the anode. In order to ensure that the reference electrode does not make direct contact with the anode, use a separate nylon sheet (or alternative membrane) placed on top of the reference followed by a few drops of water, a small amount of coating paste (repeat steps 3.2 – 3.3) and then the anode.
   4. Connect the leads to the electrodes, seal, press, begin coating and monitor changes in voltage or current.

5. Characterization with quantum yield fluorescence spectroscopy

1. If the coating contains fluorescent composite particles, use a fluorometer equipped with an integrating sphere to obtain absolute quantum yield measurements.
   1. Place the coating into the fluorimeter stage with the fluorescent coating facing 45° from the excitation source and 315° from the detector.
   2. Record the fluorescence spectra starting at a wavelength below the excitation wavelength to record the areas of the excitation peak and the fluorescence peak.
   3. Remove the sample from the fluorometer, and repeat step 5.1.2 to record the blank excitation peak. Calculate the quantum yield (QY) from the ratios of the areas of the excitation and emission peaks (Equation 3 and Figure 3B).
          Equation 3
      where A_em, A_ex and A_o are the peak areas at the emission wavelength of the sample, the excitation wavelength of the sample and the excitation wavelength of the blank, respectively.

6. Characterization with optical microscopy

1. Place the sample on the stage of a calibrated optical microscope with the coating side facing the lenses and bring the surface into focus.
2. Record surface images at the desired magnifications and surface sample sites.
3. Using image analysis software (ex. ImageJ (IJ 1.46r)), calculate and plot the surface coverage and average composite particle size.
4. To determine coating thickness and cross-section features, cut the cathode foil with scissors. Set the coating on the side and re-adjust the focus. Repeat steps 6.2 – 6.3.

Results

After following this protocol, a thin coating of metal should become plated onto the cathode surface and contain the composite particles that were added to the coating paste. Fluorescent or colored particle incorporation can be observed by visual inspection as a result of a change in appearance compared to the uncoated surface (Figure 1A1-A3). To investigate the percent surface coverage of the composite particles and to observe the surface morphology of the coating, optical ...

Discussion

Critical steps of electrostamping. Bathless electrostamping shares many of the same critical steps with traditional bath electroplating. These include proper cleaning of the electrodes, mixing metal ions into the electrolyte and applying and external or chemical (electroless plating) potential to cause reduction of metal onto the cathode. In addition, the oxidation of the anode and cathode should be avoided after acid activation by quickly rinsing with water and adding these electrodes to the setup.

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Materials

Name	Company	Catalog Number	Comments
37% M Hydrochloric Acid (aq)	SigmaAldrich	320331-500ML	corrosive - handle in fume hood
70% Nitric Acid (aq)	SigmaAldrich	438073-500ML	corrosive - handle in fume hood
Barium magnesium aluminate, europium doped (s)	SigmaAldrich	756512-25G	fine powder
Boric Acid (s)	SigmaAldrich	B6768-500G	toxic
Cotton Swab	Q-tips	Q-tips Cotton Swabs
ImageJ	National Institutes of Health	IJ 1.46r	free software
Nickel (II) chloride hexahydrate (s)	SigmaAldrich	223387-500G	toxic
Nickel (II) sulfate hexahydrate (s)	SigmaAldrich	227676-500G	toxic
Nickel foil (s)	AliExpress	Ni99.999
Nitrile gloves	Fisher Scientific	19-149-863B
nylon membrane (s)	Tisch Scientific	RS10133
Optical Microscope equipped with FTIC filter (470 ± 20 nm)	Nikon	Eclipse 80i
Plastic Wrap	Fisher Scientific	22-305654
Porcelain Mortar	Fisher Scientific	FB961A
Porcelain Pestle	Fisher Scientific	FB961K
Potassium Hydroxide (s)	SigmaAldrich	221473-25G	corrosive
Potentiostat with platinum wire	Gamry Instruments	1000E
Scoopula	Fisher Scientific	14-357Q
Spectrofluorometer	Photon Technology International	QM-40
Strontium aluminate, europium and dysprosium doped (s)	GloNation	756539-25G	powder
Variable linear DC power supply	Tekpower	TP3005T
Yttrium oxide, europium doped (s)	SigmaAldrich	756490-25G	fine powder